Gale Warning 40, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
155 PM AKST Mon Jan 27 2020

.SHORT TERM...Very active and high impact weather pattern
continues in the short term. High winds, heavy snow and rain, and
very high seas are expected over the next 48 hours and potentially
longer.

Currently SE AK is between systems as a large upper trough remains
centered over the western Gulf. Expansive cold airmass is noted
over the western and central Gulf and some of this air will be
streaming eastward over SE AK over the next 24 hours. This cold
air will be modified somewhat by its traverse over the warmer Gulf
waters and thus will not be as deep or as cold as the airmass that
was in place ahead of the system from a couple of days ago. A
shortwave rotating around the main upper trough will enhance snow
shower activity through the night over the NE Gulf coast and thus
have extended the Winter Weather Advisory there through midnight to
account for this. Winds remain strong out of the south over upper
Lynn Canal this afternoon due to strong SW flow over the mtns and
the resulting lee troughing effect increasing local pressure
gradients. These winds should gradually diminish Tuesday as the
next storm approaches from the S.

Attention then turns to the vigorous PV max now seen on GOES 17
WV imagery over the NPAC along 50N 160W. All guidance indicates
that as this PV lifts into the Gulf on Tuesday, that some degree
of phasing will occur between it and the polar trough over the
western Gulf. Surface low will deepen to around 955 mb on Tuesday
over the western Gulf. Rapidly occluding and storm force front
will move into the eastern Gulf Tuesday night and across the
Panhandle Wednesday morning. Surface wave looks to form on the
front and track either along or just E of 140W late Tuesday
night. This is a very favorable track for high winds over the
southern Panhandle and Gulf coastal areas. Low level wind fields
are quite impressive, with 80 to 90 kt of SE flow at 925 and 850
mb. Forecast profiles from hi-res models show 50 to 55kt as low as
500 ft near Ketchikan, Sitka, and Prince of Wales Islands late
Tuesday night. Surface pressure gradients will favor high winds
over much of the southern and central panhandle with the PASI-PAKT
gradient peaking at 16mb late Tuesday night. Storm force winds
will spread from the eastern Gulf into Clarence Strait with gales
over many of the rest of the inside waters S of Lynn Canal. Wind
gusts to 60 mph or higher are expected in land areas adjacent to
these channels so went with a High Wind Warning for a large part
of the southern and central Panhandle for this time frame.

Further north, at least some colder air will be in place so that
heavy snow will once again be an issue for far upper Lynn Canal.
60 to 80 kt of deep S'ly to SE'ly moisture transport will likely
lead to snowfall rates of 2 inches per hour at times for the
Haines and Klondike Highways. With the cold air source being a
longer trajectory over the Gulf, think that sea level Haines and
Skagway will be able to transition to a rain/snow mix by Wednesday
morning as a mix out of the cold air will occur. Further up the
highways, heavy snow will continue through Wednesday. When it's
all said and done, expect yet another 12 to 24 inches of snow for
areas already hard hit by last weekends storm. Highest amounts
will be over the upper elevations of both highways.

Confidence is high on the impact of this storm but a bit lower in
the details, such as timing of the high winds and snow to rain
changeover etc.

.LONG TERM.../Wednesday through Sunday/...The active weather
pattern appears very likely to continue in the extended range
into the weekend. The split flow pattern that is in place is
challenging in the sense that Southeast AK could experience rain
or snow, but small details that are difficult to devine in the
long term forecast are not quite available. We do know that
precipitation will likely continue to fall each day through
Saturday, and if snow does fall, it could be impactful.

Ensemble agreement is good in the overall synoptic pattern over
the Gulf of Alaska. Large upper trof will remain in place across
the western parts of AK extending over the W gulf. Early Wednesday
an upper shortwave rotating around the trof will move N across
the eastern gulf and lift across the panhandle. At the surface the
low pressure system remains and issue with a complex of multiple
waves possible as the low moves northward into the N gulf through
Wednesday. Low level temperatures will briefly warm as the front
lifts north across the panhandle early Wednesday, but colder air
is expected to wrap in behind the low and across the E gulf and
panhandle later in the day. Currently expecting this system to
provide a rain/snow mix across the Icy Strait corridor, including
Juneau, while the more impactful snowfall is likely across the N
Lynn areas including Haines and Skagway where temperatures will
likely remain near or below freezing.

Another wave will move NE out of the NPAC into the SE gulf by
Thursday morning. This system has the potential to the be an
impactful wind event where and if it makes landfall across the
panhandle. Operational models all depict a rapidly developing low,
but handle the details very differently. The placement of the low
will also determine where impactful snow will be possible
Thursday. Did increase winds to gales in the E gulf but future
work will need to be done with better consensus. By Friday, there
is more disagreement between the models, but still expect a
surface low to be churning in the N gulf through Saturday. By
Sunday there is a chance for a brief break as a ridge moves east
across the gulf and panhandle. Overall forecast confidence is
average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...High Wind Warning from 6 PM Tuesday to 3 AM AKST Wednesday for
     AKZ023-026>028.
     Winter Storm Warning from 6 PM Tuesday to 6 PM AKST Wednesday
     for AKZ018-019.
     Strong Wind until midnight AKST tonight for AKZ018.
     Winter Weather Advisory until midnight AKST tonight for AKZ017.
MARINE...Gale Warning for PKZ041>043-051-052.
     Small Craft Advisory for PKZ012-021-022-031-033-036-053.

&&

DEL/DDH

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